1. Field of the Invention
This invention generally relates to an improvement in the field of suspensions and drive systems for variable power dividers for spacecraft, and more particularly but not by way of limitation to a mechanical arrangement that drives a microwave variable power divider to a predetermined position with minimum friction loss and holds it in such position until the position is to be changed.
2. Description of the Prior Art
This invention has as its main purpose the provision of an essentially friction free drive which has an inherently high reliability when used for a microwave mechanical variable power divider in spacecraft applications. In modern communications spacecraft, the antenna system typically includes a number of feed horns arranged in a predetermined array which may directly illuminate the earth, or may be used as a source to feed a parabolic reflector which illuminates the earth. By suitably energizing the elements of the array with the correct amounts of power, the beam or beams from the antenna system can provide small or large spots of either circular or non-circular shapes. Thus, the utilization of the radio frequency ("RF") energy that is available can be made more effective for communication purposes.
In practice, the adjustments of the power levels to the feed horns are made either by switches and fixed power dividers (which permit only a predetermined set of fixed radiation patterns) or by means of variable power dividers ("VPD") which permit a much higher degree of control of the beam patterns. The changes in the beam patterns are made by remote control from the earth whenever the system operator deems such changes to be appropriate. Thus, controllable and stable VPDs are essential elements of such systems.
In the past, VPDs have comprised ferrite devices which have the property of producing phase shift of signals in a microwave waveguide, the amount of shift being controlled by an externally applied magnetic field. In one embodiment these devices comprise a Faraday Rotator while in another, the phase shifters are used with microwave hybrid junctions to add or substract power in a controlled manner. Mechanical devices have also been proposed, but have not been accepted largely due to a question of reliability of bearings and drive mechanisms which may fail due to friction, loss of lubricant, sticking, and other problems associated with mechanical devices.
However, mechanical devices are generally considered to be superior in performance to the ferrite devices in that they can be expected to have lower RF losses and have inherently higher stability during temperature changes and with the passage of time. Further, when mechanical devices are properly designed, they need power only to change the state from one division ratio to another, a property not shared by some of the ferrite devices.
Accordingly, it is a purpose of this invention to provide a mechanical device which (i) virtually eliminates the friction commonly encountered in mechanical devices, and (ii) provides both a motive force arrangement to drive the VPD to a predetermined setting and a holding force which keeps it at that setting until it is to be changed again.
Prior art patents relating to electromagnetic brake arrangements include U.S. Pat. Nos. 4,066,152; 2,340,052; 2,312,077; 3,896,925; and 1,677,024 but none disclose the mechanical structure of the present invention that drives a VPD to a predetermined setting and holds it in such setting. U.S. Pat. No. 3,487,281 relates to electromagnetic circuitry for controlling the rotation of a rotor in a electric motor.